When a mobile communication system transmits and receives packets, a receiver must inform a transmitter of the success or failure of packet reception. If the success of packet reception is decided, the receiver transmits an acknowledgment (ACK) signal to the transmitter, such that the transmitter starts transmitting new packets. Otherwise, if the failure of packet reception is decided, the receiver transmits a non-acknowledgment (NACK) signal to the transmitter, such that the transmitter retransmits old packets (i.e., previous packets) related to the NACK signal. This operation is referred to as an automatic repeat request (ARQ).
This ARQ operation is combined with a channel coding scheme, such that a hybrid ARQ (HARQ) is proposed. The HARQ scheme combines each retransmitted packet with pre-received packet, and reduces an error rate, such that it can increase the efficiency of an overall system. In order to increase a system throughput, the HARQ requires faster ACK/NACK responses from the receiver as compared to the conventional ARQ operation. Therefore, the HARQ scheme transfers the ACK/NACK responses according to a physical channel signaling scheme. The HARQ implementation method can be generally classified into two methods. A first method is a Chase Combining (CC) method, and allows retransmitted packets to have the same modulation method and the same coding rate in the same manner as in pre-transmitted packets. A second method is an incremental redundancy (IR) method, and allows such retransmitted packets to have another modulation method and another coding rate different from those of the pre-transmitted packets. In this case, a receiver can increase a system throughput by means of a coding diversity.
A multicarrier cellular mobile communication system allows terminals contained in one or more cells to transmit uplink data packets to a base station. The terminals are able to transmit uplink data packets in one subframe, such that the base station should be capable of transmitting ACK/NACK signals to the terminals in a subframe also. If the base station performs CDM-multiplexing within some time-frequency domains of a downlink transmission band of the multicarrier system on several ACK/NACK signals transmitted to terminals within the single subframe, ACK/NACK signals about other terminals are distinguished by an orthogonal code or a Quasi-orthogonal code multiplied in the time-frequency domain. Also, if QPSK transmission is carried out, the above ACK/NACK signals can be distinguished from each other by different orthogonal phase components.
If each ACK/NACK signal is CDM-multiplexed and transmitted to transmit several ACK/NACK signals within a single subframe, there are needs for downlink radio channel response characteristics not to be greatly changed in a time-frequency domain because orthogonality between different CDM-multiplexed ACK/NACK signals is maintained, such that a satisfactory reception throughput can be acquired although a specialized reception algorithm such as a channel equalizer is not used.
CDM multiplexing of the ACK/NACK signals must be carried out in a time-frequency domain in which a radio channel response is not greatly changed. However, if a radio channel quality of a specific terminal is poor in a time-frequency domain to which the ACK/NACK signal is transmitted, an ACK/NACK reception throughput of the terminal may be greatly deteriorated.
Therefore, the ACK/NACK signal transmitted to an arbitrary terminal within a single subframe is repeatedly transmitted over time-frequency domains distributed on several time-frequency axes. The ACK/NACK signals transmitted to other terminals are CDM-multiplexed in each time-frequency domain, such that a reception end is able to acquire a time-frequency diversity gain in receiving the ACK/NACK signals.